Vehicle HVAC systems typically use a cylindrical, centrifugal blower which is driven about a central axis and contained within a scroll housing. The scroll housing is essentially coaxial to the blower axis, although its side wall is arrayed in a spiral of steadily increasing radius. At least the top wall of the scroll housing is substantially flat, and open to the center of the blower, so that it can physically mount an air inlet assembly, which is the most upstream structure of the system. The air inlet assembly is typically a box like structure, sometimes peaked on top like a house roof, with a fresh air opening that faces toward the underhood space of the vehicle, and an opposed recirculation opening facing toward the vehicle interior. A flapper door swings back and forth, inside the inlet, selectively opening or closing the fresh air or recirculated air inlet to the system. Basically, a box shaped inlet is mated to a cylindrical blower and housing, which does not make for a compact, low profile assembly overall.
A relatively recent development in HVAC systems is the introduction of air filters, sometimes called pollen filters. These are usually flat structures, sometimes broken up into several pieces, which slide fit into the air duct work wherever there is access. Access is often difficult, requiring the use of dedicated access openings through the instrument panel. Access is often better just upstream of the fresh air inlet, under the hood, with the obvious drawback that only fresh inlet air is filtered. While the interior of air inlet assembly would be a desirable location for a filter, since it would be able to handle both fresh and recirculated air, the presence of the interior flapper door generally prevents such a filter location. One proposed design, disclosed in co assigned U.S. Pat. No. 5,391,112, changes the traditional box shape to a semi cylindrical shape, although it is not coaxial to the scroll housing. A door swings over the outside of the housing, allowing for a U shaped filter internal to the housing, where it can filter both air inlet flows. The filter is a flat piece, inserted through one opening of the inlet and bent into a U shape, providing approximately 180 degrees of filtering surface. The inlet is not significantly lower in profile or more compact than a conventional box shaped inlet, however.